Sedimentary organic carbon and nitrogen contents and their isotopic compositions along two transects on the East China Sea shelf

Surficial sediment samples were collected from two transects on the shelf o f the East China Sea: one across the shelf and the other along the coast, b oth starting near the Changjiang river mouth. They were analyzed for conten ts of total nitrogen (TN%) and total organic carbon (TOC%) and for their is otopic compositions (delta N-15 and delta C-13(TOC)). The TOC% and TN% valu es varied concomitantly in the ranges of 0.2 to 1.5% and 0.02 to 0.19%, res pectively. The delta C-13(TOC) value ranged from -25.1 to -19.6 parts per t housand. The lower delta C-13(TOC) values (<-21.5<parts per thousand>) all occurred in the inner shelf corresponding to higher TOC% (>0.5%). Away from the coastal zone, TOC% dropped sharply with corresponding increase in the delta C-13(TOC) value. The delta N-15 value, varying from 3.6 to 4.9 parts per thousand, displayed a more complicated distribution pattern. Distinctiv ely lower delta N-15 values (<4. 1<parts per thousand>) occurred near the C hangjiang river mouth and near the shelf edge. Isotopically light riverine nitrogenous materials and preferential uptake of isotopically light nitroge n in the rich supply of nutrients in the Changjiang plume may have contribu ted to the observed low values near the river mouth, while nitrogen fixers in the Kuroshio waters may have contributed to those near the shelf break.